I would go with Heidfeld. Had he landed a drive seat in McLaren, when he had a chance, maybe things would have gone other way. Now we have a guy who’s better than half of the current “best ever” grid and doesn’t have a drive. Damn you, paydrivers!
Chris Amon is the classic, he was considered by his contemporaries to one of the top drivers (his contemporaries being Dan Gurney, Jack Brabham, Jim Clark, Jackie Stewart etc!).
Stefan Bellof was one of the all time great talents, he truly was on a par with Senna and Prost in terms of pure speed. He was the best not to win a Grand Prix for me.
Other honorable mentions are Ricardo Rodriguez, Tom Pryce, Vic Elford (who was and still is one of the best all rounders ever seen in 4 wheeled motorsport) and I honestly think of the last 20 or so years, Nico Rosberg is probably the best to have not yet won a race.
It’s nice to see that Vic Elford is mentioned. Like Amon he was quite unlucky (his Porsche broke while he was leading Le Mans in 1970 a few hours before the finish). As far as I know, he’s the only driver to win the Monte Carlo rally and to have competed in the Monaco GP in the same year (1968). But I am not sure he was GP winner material, as much as I admire him.
There are lots of good names here but I have to pick Stefan Bellof as well. He looked to be an outstanding talent. I remember when I first watched the 1984 Monaco GP, I immediately had to Google him afterwards because I couldn’t believe that I had never heard of him before. His death was a great loss to the sport, imagine how epic the late 80s could have been if he had been around to challenge the like of Mansell, Senna and Prost.
Warwick was offered a William seat for 1985 but decided to stay at Renault (bad choice). Mansell got the seat at Williams and the rest is history. Ayrton Senna didn’t want him at Lotus in 1986 for fear he would outpace him. Warwick made some bad choices in his career and only once got a car matched to his potential: at Renault in 1984 with two second places and two third places to show for in a year when the McLaren were the class of the field. Not bad for a “journeyman”.